Vector assignment flawed

[Garth Wells]

Current vector assignment is based around duplicating vectors rather than assigning values. We need to support three 'types' of assignment:

1) Assign just values for equal-length vectors. This can work using GenericVector::get_local and GenericVector::set, and should permit the assignment of values to a vector for a vector based on a different backend

2) Copy values for equal length vectors with identical parallel layout (VecCopy in PETSc). This is fast and is supported by backends. Typical use would be updating a vector in a time dependent or nonlinear problem

3) Assign entire object to another. This is duplication (VecDuplicate, followed by VecCopy in PETSc).

A question is what is the best interface for this?